New to the forum. It rocks. First toyota, recently purchased.

1990 pickup, 6cyl, 5spd, 218k miles.

Recently replaced fuel tank, fuel pump bracket and fuel sending unit due to rust leaks. This was causing a serious studder as the air that was getting into the fuel lines passed through the system.

Previous owner had a leaking valve cover but fixed it before selling. Otherwise unsure of maintenance history.

I have a few problems.

The truck lurches coming both on and off the gas so that it's impossible to cruise at anything less than highway speeds. Also, there seems to be a power hesitation at times, occasional jumps or drops in RPM, and a loud almost sound hard to describe at 1200-2000 RPM. I'll call it a thwacking buzz. :s

I believe the previous owner had the idle cranked, perhaps to deal with this driveability issue, because the lurching started when my mechanic changed the idle. But I'm not sure what it was idling at before. With the screw out about 3/4 turn, it's idling nicely at 800-1000 RPM or so.

Three discoveries so far:

-I found is the dashpot bellows is all cracked and the piston is slow to come all the way out.

-Resistance tests on the TPS check out, although it may need adjusting since, just by feel, it seems to me that the throttle lever is much further from the stop screw than 0.5mm when the TPS starts to show resistance. Maybe 1.5mm! I have not taken the TB off yet to try to adjust this. (The coolant lines are giving me trouble to remove).

-Finally, the AFM pins E1-FC give zero ohms when the truck is parked. If I understand correctly, that means the measuring plate isn't closing when it should be.

Questions:

-are the dashpot and AFM enough to explain my symptoms?
-am I right to attempt to take off the AFM and see if I can get the plate to close before replacing it?
-how critical might that TPS adjustment be?
-should I be worrying about vacuum, timing, or fuel delivery issues at this point?

I ask because I have to make some decisions about if/how to get all this work done.

Thanks, this forum has taught me a lot so far!

Paul

 

Paul,

1. The dashpot is only for reducing smog emissions for letting off the gas and for a "smoother" deceleration. I've gone through two of them. It's just there for when you let off the accelerator completely the throttle body doesn't snap shut and make the truck jerk slightly. Only happens in 1st-3rd gear for me and it's negligible. I've driven with no dashpot with no issues.

The AFM may be your problem. I'm not up to speed on the ohm reading but if you measured it and it's not matching what the service manual says, there's an issue. Whether it's your lurching problem is yet to be seen.

2. I'd try this last. New AFM's cost about $250 and junkyard ones are sometimes unreliable.

3. TPS adjustment on this engine is huge. Too big of a gap could cause the engine to think it's in "idle" until you're hitting 1.5k or 2k RPMs. This could be your issue.

4. Yes, worry about vacuum. Look over some vacuum diagrams and make sure the previous owner or a mechanic didn't cross up the vacuum lines. There's a ton of them.

None of these are hard, just takes patience. You should be able to check it all with the FSM, a multimeter and a feeler gauge ($5).

 

Thank you! This really struck me as relevant to my symptoms, so I redoubled my efforts to adjust the TPS. I managed to do it without removing the throttle body (by using a No.1 Phillips drill bit and vice grips). Adjusted it following 4crawler's article, using a slice of credit card between the throttle lever and stop. I later bought feeler guages, but the 0.71 was identical to the card. And I couldn't have fit the guage back there anyway! (Just hints for other newbies.)

Now she idles and purrs like a kitten and drives smoothly, at least in terms of the lurching. YAY. One problem solved.

Drove it around a little, and got CELs, pulled the codes:

Code 21 Main oxygen sensor signal fault
Code 24 Intake air temperature signal fault
Code 25 Air/fuel ratio LEAN
Code 26 Air/fuel ratio RICH
Code 31 and 32 Air flow meter circuit or Vacuum sensor signal fault

And I still have some driving issues:

-that weird sound which I will try to record, now less frequent and confined to narrower rpm ranges (makes me think incorrect air/fuel ratio).
-very occasional obvious losses of power (have to downshift and limp along)
-subtle hesitation or sluggishness
-trouble holding 60-70mph up typical freeway hills

Now I don't know how powerful to expect this truck to be (it's lifted a little with 31x10.50R15LT Avalanche X-treme tires), but I'm not flogging it, the rpms just won't rise at times, so it seems to me like it's not getting enough fuel.

Could this still be the AFM (measuring plate not closing)? Worth taking off? Can a vacuum leak cause intermittent power loss? Other things that come to mind are injectors, fuel pump, and coolant lines (thus engine temp sensor)?

I can love my truck again, so I will be working through some checks one by one, but any suggestions welcome!

 

Have you checked timing? Surging can be caused if someone loosened the distributor bolt and turned it for some reason. Worth checking, after all, it's free to set time usually.

 

[QUOTE=Neeaar;52099558]
Code 21 Main oxygen sensor signal fault
Code 24 Intake air temperature signal fault
Code 25 Air/fuel ratio LEAN
Code 26 Air/fuel ratio RICH
Code 31 and 32 Air flow meter circuit or Vacuum sensor signal fault
[QUOTE]

That's a lot of codes ! If you can, take off the negative clamp on the battery for a few minutes and reset the ECU. I have a feeling some of those code were "stored", meaning they happened once a while ago but aren't relevant to right now. Drive it until the light comes back on and then pull again.

Codes 24 and 25 are conflicting, you can't have both at once.

 

cel codes 24, 31 & 32 air intake temp sensor/afm signal etc... AFM is toast or the wires/connesctor between the afm and ecu are toast.

code 21... either your o2 sensor is junk, or the wires/connector between it and the ecu are junk. Fix those 2 issues, and see where you go from there.

 

Hmm, no symptoms I would describe as surging anymore; the lurching was fixed by adjusting the TPS.

I'm pretty sure the mechanic cleared them all and they're new.

If the ECU wasn't sure how much fuel to inject, couldn't it end up screwy in either direction? (I don't know, I'm just guessing, asking.)

I'll follow up on the AFM today, including wires, and report back.

 

Progress.

I tested the AFM and got 0 between FC-E1 (should be infinity). Took off the AFM, started moving the plate, and then I was suddenly getting infinity between VS-E2, whereas before it was on spec (200-600). Checked FC-E1, and it was infinity, but staying infinity when I moved the plate. I noticed a rattle in the unit...

Took the cover off the contacts and hello! the lead wire on the sweeping contact had fatigued at the spade terminal. Soldered that and tested all within spec.

Reasembled, reset codes, had to adjust idle down a bit, took it for a drive, still getting that funny slappy hammering around 1000rpm (thwacking buzz, as I called it earlier), but no noticeable power issues. Got code 21 after just a few minutes of driving.

I don't have an analogue multimeter with me, but the digital multimeter showed 117mV on Vx1, and only 8mV on Ox1. I understand Ox1 should be 400-900mV, but what should Vx1 be? The same?

So, I think I have a bad 02 sensor, right?

Thanks! brand new to fixing cars but this is becoming fun.

 

Pulled my O2 sensor, and found a severed wire! Another easy fix.

Since the TPS adjusmet, AFM and O2 sensor wiring jobs, it's now driving very well, has power like never before, no sluggishness, no problems on the highway, a whole new beast! So excited, all free fixes!

I drove about 60 miles home from the cottage last night with no codes. Went to dinner, then on the drive home got Code 25, and 26, LEAN and RICH!

This morning took a drive and did experience one loss of power, not the subtle sluggishness but obvious sluggishness that it suddenly snaps out of.

And I still have that noise, particularly when it's dropping through around 1000rpm back to idle when I coast. I will put it on youtube soon if I can't figure it out.

So I am still hunting some gremlins.

-any thoughts on how I can be too LEAN and RICH?
-still wondering whether it's possible to have intermittent vacuum leaks, or vacuum leaks that cause intermittent problems..
-next tests?

 

LOVE. THIS. FORUM. Learning tons as I search and search...

Still struggling with this engine noise:
notes:

-have had the noise since getting the truck, but when first purchased it seemed to go away after the truck warmed up. at that time, the TPS was set switching in and out of idle at 1800 rpm! and the idle was cranked, probably to compensate for the resulting lurching.
-i have tuned the tps both by hand based on engine speed, and based on feeler guage/ohm meter, same result.
-when the idle switch on the TPS is adjusted out of range, the noise does not occur, but the truck "idles" at ~1000rpm.
-noise is coming from the passenger side near the intake manifold, afaik, as the rpms drop through 1000, when the TPS is adjusted properly.
-when the TPS was maladjusted, or differently adjusted, the noise happened at other rpms
-timing jumper doesn't drop RPM no matter where the TPS is set.
-still getting 25 and 26 codes LEAN and RICH coming together!
-no other codes
-TPS tests within all specs for resistance
-throttle body is clean and throttle stop is carefully set
-vacuum checked with basic carburator cleaner technique
-new plugs today (the ones in there had 0.045" gaps!! wtf), no change in noise
-previous owner was clearly an idiot and nothing should be taken for granted (although long term owner before that was a fastidious mechanic).

questions:

1 - how much should RPM drop when TE1 and E1 are jumpered?
2 - will I be able to check timing without that working?
3 - any ideas what I'm hearing?
4 - how do I check the continuity on the TPS-ECU wiring (where's the other end?)

 

1. Mine doesn't change. The factory service manual says is should, but mine doesn't with no problems.

2. Yeah, to check timing you just need a timing light. Just jump TE1 and E1 first, and turn distributor until it's at 10 degrees (or 12 if you like a bit of power, watch for spark knock though)

3. Sounds to me like your PAIR valve, or maybe some serious valve knock. Hard to say. Have you ever adjusted the valves or replaced the shims?

4. Your ECU is located behind the kick panel on passenger side in the cab. You'd have to look it up on the ECU, there's about 40 or so wires.

Did you adjust the TPS per the service manual or just by ear? You have to do it right with a feeler gauge and an ohmmeter or your engine will not run correctly.

 

Was away, finally had a chance to poke around again. The hose that travels under the intake from the air cleaner was disconnected from the PAIR valve (if I've got that rectangular box named correctly). Fixed!!

Still planning to do a timing check and a more thorough vacuum check. I've got an oil leak to. :S

Thanks all, special thanks to Gamefreakgc!

 

Aww, shucks. Funny thing is, I was in your shoes just three years ago. Careful, once you catch "the bug" it never stops...

Oil leaks are normal. You've got an old engine and gaskets and rubber only last so long. Unless it's massive and you find yourself filling it with oil in between changes, don't worry about it. However, make sure it's engine oil. My rig also leaks power steering fluid (no biggie) and for a time was leaking differential fluid from the axle (bad). I also have a clutch line that leaks, but hardly worth noting. Years of grease have plugged up the leak lol!

 

haha just watched your video and I thought to myself THATS A PAIR VALVE!
You should see how some customers explain that noise

Glad to see you figured that one out LOL

 

I would call the problem fuel injectors. I just put my third motor in a 90 4 runner. Have swapped injectors once due to vehicle sitting for a period of time. Need to swap again now as I continue to drive it it is having same symptoms as the first time.

 

hesitation? rich and lean codes at the same time?

its probably injectors.

 

I had this problem ...What kind of voltage do you get on the NE pin at the ECM ?

 

Since there's been more action here I thought I should recap. Sorry to be long-winded but maybe it will help someone.

My problems were

-fuel pump hanger leak (I thought this was letting air into fuel line and causing massive hesitation that would clear up, more on that in a minute)
-maladjusted TPS (caused lurching, no associated CEL)
-a bad connection in the AFM (caused poor power, threw CELs 24 and 31)
-severed O2 wire (threw CEL 21).
-"slappy buzz" on deceleration and cold idle that turned out to be a vacuum hose off on the reed valve in the PAIR system.

All fixed. Also changed plugs, air filter is clean, been running injector cleaners, and cleaned O2 sensor at the same time.

After fixing all those things, truck was running pretty well except for feeling intermittently extra sluggish or extra peppy. I was still getting CELs 25 and 26 at the same time, and recently the massive hesitation returned, the one I thought was being caused by the fuel leak... :s

Well I can't find any fuel leaks. I had already ordered a new O2 sensor, not knowing how old it is and suspecting it to be causing the codes and sluggish/peppy issue. In the meantime, I thought I would look at my O2 sensor again. Found a wire had worn through a little bit on the heatshield where I had tucked it behind for "safe keeping" (bad idea)! Note that I didn't get another CEL 21. Fixed that, cleaned it again, and filed the mating surfaces and made a makeshift gasket out of boxboard (it didn't have one!!). Massive hesitation gone, and new O2 sensor on the way.

The lesson is that bad wiring, old O2 sensor, and maybe the exhaust leak at the sensor can wreak havoc on the fuel trim. I also believe that it's responsible for the 25/26. If it was injectors, wouldn't it run rough all the time? As it stands, I think the bad wiring was causing the massive hesitation and then the ECU would go back into open loop mode.

I will report again with the new O2 sensor and gasket.

__________________________________________________ _______________

On a side note, I have a leaking cab, so I wondered if the ECU could be problematic. The previous owner had left an ECU with the truck, so that was suspicious. Opening up the ECU I found the covers had corroded and a little chunk of something was sitting on a couple of the pins. Removed that and cleaned contacts. Probably not related, but worth checking. For those doing ECU testing, my 1990 3vze 4x4 5spd computer does not have the same pinout as what is shown in the ubiquitous 1993 FSM. The pins are labeled on the circuit board if you take the covers off. Turns out the extra ECU was the wrong part, so he probably was thinking of swapping it and couldn't.

About my oil leak, it's definitely the valve covers. The previous owner was supposed to have fixed them, so when I tried tightening the bolts, two of them were already stripped, so I didn't even touch the rest until I figure out what to do.

I just changed all my gear oils and shifting is noticeably better.

Still planning to check timing and valve clearance as I learn more, and when I have the valve covers off for fixing. I'll also do the PCV/grommet.

One other interesting problem. Attendants around here top-off the fuel tank, and one time after a fill, in the hot sun gas was coming back out the fill-tube. I had to open the fill plate and rock the truck to release the air lock. Happened a few times that day. So I took out the vapor charcoal canister and sure enough one of the one-way valves was blocked, but blowing into it I was able to get it open again. Also won't be letting anyone overfill the tank any more.